The invention relates to certain acetylenic di(t-amyl peroxides) useful as cross-linking/blowing agents and, in particular, to their use in compositions and processes for the preparation of foamed, cross-linked polyethylene.
It is known to form foamed, cross-linked polyethylene using chemical blowing agents by heating an expandable mixture comprising a thermoplastic resin, such as polyethylene, a cross-linking agent, and a chemical blowing agent. U.S. Pat. No. 3,470,119 teaches a method for manufacturing foamed polyethylene by use of a chemical blowing agent. U.S. Pat. No. 3,658,730 teaches a specific composition for blowing thermal plastic resins. Chemical blowing agents (CBAs) are generally solid compounds or compositions which decompose at a specific temperature to yield a large volume of gas. Sodium bicarbonate, the most widely used inorganic CBA, has limited use because it decomposes only in the presence of an acid.
Organic CBAs tend to evolve gas over a more defined temperature range. Modern Plastics Encyclopedia, 1979-1980, pp. 186-188, teaches a group of known organic CBAs useful in the production of foamed, cross-linked polyethylene. The blowing composition of U.S. Pat. No. 3,658,730 utilizes azodicarbonamide, a known organic blowing agent, in conjunction with a chromium compound. The use of azobisformamide and many other blowing agents is taught in U.S. Pat. No. 3,470,119. The disadvantage of using such azo blowing agents is the production of small amounts of ammonia, which can corrode equipment.
The production of cross-linked polyethylene foams generally requires the use of cross-linking agents in addition to the blowing agent. An exception to the requirement of separate cross-linking and blowing agents is disclosed in U.S. Pat. No. 4,129,531 and U.S. Pat. No. 4,101,464, wherein certain azo esters function as cross-linking/blowing agents. However, as with other azo blowing compounds, small amounts of ammonia are produced. A second exception to the requirement of separate cross-linking and blowing agents is disclosed in U.S. Pat. No. 4,276,247, wherein cyclic peroxyketals are used as combined cross-linking/blowing agents in the production of foamed polyethylene.
Many organic peroxides, including di(alkylperoxy) acetylenes, are known to be effective cross-linking agents for thermoplastic resins. U.S. Pat. No. 3,214,422 teaches di(t-butylperoxy)-hexynes and -octynes as cross-linking agents for polyethylene; this patent does not disclose any possible use of these compounds as blowing agents.